1. Field of the Invention
The present invention generally relates to an image forming apparatus for forming an electrostatic latent image on an electrophotographic photosensitive member by electrophotography and visualizing the electrophotographic latent image with a developer, an image bearing member life detecting method, and a process cartridge detachably attachable to the image forming apparatus.
The electrophotographic image forming apparatus includes, e.g., an electrophotographic copying machine, electrophotographic printer (e.g., LED printer or laser beam printer), and electrophotographic facsimile apparatus.
The cartridge detachably attachable to the main body of the electrophotographic image forming apparatus means a cartridge having at least one of an electrophotographic photosensitive member, an electrostatic charging means for electrostatically charging the electrophotographic photosensitive member, a developing means for supplying a developer to the electrophotographic photosensitive member, and a cleaning means for cleaning the electrophotographic photosensitive member. Among detachably attachable cartridges, a process cartridge is a cartridge which is an integral unit of an electrophotographic photosensitive member and at least one of an electrostatic charging means, developing means, and cleaning means, and is detachably attachable to the main body of an electrophotographic image forming apparatus, or which is an integral unit of at least a developing means and electrophotographic photosensitive member and is detachably attachable to the main body of an electrophotographic image forming apparatus.
2. Related Background Art
Conventionally, an electrophotographic image forming apparatus such as an electrophotographic copying machine or laser beam printer uniformly electrostatically charges an electrophotographic photosensitive member by using an electrostatic charging means, and irradiates the surface of the photosensitive member with light corresponding to image information to form a latent image. Then, the apparatus supplies a developer to the latent image by using a developing means to visualize the image, transfers the visualized image onto a recording medium, and fixes the image by a fixing apparatus to obtain the image. After transfer, the electrophotographic photosensitive member is cleaned by a cleaning means.
This image forming apparatus adopts a process cartridge method in order to facilitate replacement and maintenance of expendables such as an electrophotographic photosensitive member and developer. More specifically, an electrophotographic photosensitive member, a developing means, electrostatic charging means, and cleaning means as process means which act on the electrophotographic photosensitive member, a developer storage vessel, and a waste developer vessel are integrated into a cartridge. The cartridge is detachably attachable to the main body of the image forming apparatus.
According to the process cartridge method, the apparatus can be maintained not by the serviceman but by the user. For example, when the developer runs out or the life of the photosensitive drum has expired, the user can replace the cartridge with a new one and form an image again. This process cartridge method can significantly improve the operability of the apparatus, and is widely used in electrophotographic image forming apparatuses.
The process cartridge type image forming apparatus must warn the user that, e.g., the life of an expendable element such as an electrophotographic photosensitive member or developer has expired or will expire soon, and allows the user to replace the expendable element with a new one at arbitrary time.
The following conventional life detecting methods for a cylindrical electrophotographic photosensitive member, i.e., photosensitive drum, are available.
(1) The number of image forming sheets is integrated to detect the life of the photosensitive drum. According to the simplest method, each of image forming recording media of, e.g., different A4 and A3 sizes is counted as one sheet. In this case, however, the life of the photosensitive drum is detected with poor precision. Also when the number of image forming sheets is merely integrated, the life of the photosensitive drum is detected with poor precision because the rotation period of the photosensitive drum per recording medium changes depending on the number of image forming sheets per job, i.e., the successive number of recording media for forming an image after the start of image forming operation, and the life of the photosensitive drum (to be described later) changes depending on the rotation period.
(2) Japanese Patent Application Laid-Open No. 4-51259 discloses a conventional method of detecting the electrostatic charge amount of a photosensitive drum by a surface potential sensor. According to this method, a decrease in the electrostatic charge potential of the photosensitive drum or a decrease in latent image contrast can be actually detected directly by the surface potential sensor. Compared to method (1), the life can be detected with high precision while the output image state is reflected. Practicing this method, however, requires a surface potential sensor and an electrical circuit for processing an output from the sensor, resulting in high cost. As for the longitudinal direction of the photosensitive drum, determination is made based on only information on the photosensitive drum that corresponds to the sensor position. The detection ability is poor for a partial error, and the stability is low due to variations or changes over time in the surface potential sensor. The life of the photosensitive drum is not always accurately detected.
(3) As a method of solving the problems of method (1) and increasing the life detection precision for the photosensitive drum, Japanese Patent Application Laid-Open No. 5-188674 discloses a method of integrating not the number of image forming sheets but the number of turns of the photosensitive drum. Some methods integrate the rotation period of the photosensitive drum on the basis of the same principle. In any method, the number of turns (rotation period) is large for a large recording medium size in correspondence with the recording medium size in one image formation, and is small for a small recording medium size. A photosensitive drum life detection error by a difference in recording medium size is reduced in comparison with the case of integrating the number of image forming sheets. Since the number of turns (rotation period) of the photosensitive drum is directly integrated regardless of the number of image forming sheets per job, the life detection precision is relatively high.
As a method developed from method (3), Japanese Patent Application Laid-Open No. 4-98265 discloses that the number of turns of the photosensitive drum is integrated in actual image formation by integrating the number of turns of the photosensitive drum only when a transferring charger as a transferring means operates, and that the life of the photosensitive drum can be more accurately detected. Japanese Patent Application Laid-Open No. 6-180518 discloses that the number of turns of the photosensitive drum while the photosensitive drum undergoes electrostatic charging processing, and the number of turns of the photosensitive drum while a cleaning member for cleaning the photosensitive drum is in contact with the photosensitive drum are respectively integrated, and the life of the photosensitive drum is determined based on the comparison with their set values (lives).
The following method is known as a method of notifying the user of the replacement timing of a process cartridge. According to a method disclosed in Japanese Patent Application Laid-Open No. 5-333626, the user is notified of the replacement timing of a process cartridge constituted by a cleaner (cleaning means) and electrophotographic photosensitive member on the basis of the life of the electrophotographic photosensitive member. The number of image forming sheets is integrated, and when the assured life of the electrophotographic photosensitive member has expired, the apparatus stops and cannot be used. As replacement display operation based on the life of the electrophotographic photosensitive member, the user is reminded to prepare for a replacement cartridge by the end of the assured life by displaying that the replacement time is coming soon, and if the cartridge is kept used, the user is warned that the apparatus will stop soon.
This prior art also enables notifying the user of the replacement timing of the cartridge on the basis of the toner amount in the recovery toner storage portion of the cleaner. More specifically, the ON period of a toner replenishment driving motor is integrated, and the apparatus stops at the earliest integration time at which conditions are supposed to become worse in consideration of variations. Also in this case, as replacement display operation based on the amount in the toner storage portion, a display which reminds the user to replace a cartridge is performed when the integration ON period of the toner replenishment driving motor reaches a given value until the apparatus stops, or a display which notifies the user that the apparatus will stop soon is performed at advanced integration time.
According to this prior art, the operation based on the life of the electrophotographic photosensitive member and the operation based on the toner amount in the recovery toner storage portion of the cleaner are generally so set as to give priority to the number of prints, i.e., the life of the electrophotographic photosensitive member. However, the operation based on the toner amount in the recovery toner vessel functions when toner replenishment is frequently done because of an extremely high image density and the recovery toner vessel will become full earlier than the assured life (assured number of sheets) of the electrophotographic photosensitive member.
In the technique disclosed in Japanese Patent Application Laid-Open No. 5-333626, the process cartridge comprises a storage means. The total energization period of the primary charger of the image forming apparatus is written at once in the storage means by a CPU in the image forming apparatus. The subsequent energization period of the primary charger is written and saved. The storage means of the spent process cartridge is recovered and analyzed to accurately determine the total amount of a use variable, such as the current number of turns of the photosensitive drum or the discharge period of the corotron in the image forming apparatus which uses the spent process cartridge. Information of the image forming apparatus can be collected at a process cartridge replacement interval. More specifically, the number of operation cycles of the photosensitive drum, the replacement time of an ozone filter, and wear data prediction of the photosensitive drum in the image forming apparatus can be determined in replacing a process cartridge.
According to the technique disclosed in this reference, the life of the photosensitive drum is determined based on the number of image forming sheets. The life prediction precision for the photosensitive drum on the basis of the number of image forming sheets is low, as described above.
Recently, some developing apparatuses for developing a latent image formed on an electrophotographic photosensitive member use a so-called single-component developer containing toner as substantially a single main component. The single-component developing type developing apparatus need not control mixing and agitation of toner and carrier and the toner density (ratio of the toner to the total amount of toner and carrier), unlike a so-called two-component developer type developing apparatus using carrier and toner particles as main components that has conventionally been popular. The single-component developing type developing apparatus achieves a small size and low cost, eliminates any developer replacement operation, and is very effective in a printer demanding a maintenance free operation. A non-magnetic toner used as the toner of the single-component developer can realize a smaller size and lower cost because no magnet roll need be attached to a developer carrying member for carrying a developer to an electrophotographic photosensitive member.
The single-component developing type developing apparatus comprises a developer vessel (hopper) for storing a single-component developer (toner), a developer carrying member (developing roller) implemented as, e.g., a roller which is arranged adjacent to the developer vessel and carries the toner to a latent image on the electrophotographic photosensitive member, a toner supply roller which is brought into contact with the developing roller and rotates in the same direction as the developing roller, and a developer layer thickness regulation means (regulation blade) implemented as, e.g., a blade for regulating the toner amount carried on the developing roller. The toner supply roller carries the toner in the hopper to the developing roller, and the regulation blade forms a thin toner layer on the developing roller. The thin toner layer is brought into contact with the electrophotographic photosensitive member to develop an electrostatic latent image formed on the electrophotographic photosensitive member. Such an apparatus is known as a so-called contact single-component developing type developing apparatus.
When a non-magnetic toner is used as the toner of a single-component developer, the regulation blade such as an elastic blade is brought into contact with the developing roller, and a thin toner layer is formed on the developing roller by using the electric charges of the toner or the Coulomb force by frictional electrostatic charge. In this manner, the toner is supplied and carried.
In recent years, contact electrostatic charging apparatuses have widely been used as an electrostatic charging apparatus for electrostatically charging an electrophotographic photosensitive member, instead of conventionally widely used corona electrostatic charging apparatuses. The contact electrostatic charging apparatus requires a lower application bias than the corona electrostatic charging apparatus, hardly produces ozone, uses a smaller number of building components of the electrostatic charging apparatus, and can reduce the cost.
Such contact electrostatic charging apparatuses are roughly classified into two, brush and roller electrostatic charging apparatuses in accordance with the form of an electrostatic charging member used. The brush electrostatic charging apparatus suffers brush marks and inclination of bristles upon long-term abutment to the electrophotographic photosensitive member. The roller electrostatic charging apparatus has difficult problems: the roller resistance must be adjusted to obtain uniform electrostatic charge, drum contamination by bleed from rubber which forms the roller must be prevented, strict constraints are posed on the roller shape and surface property in order to obtain uniform electrostatic charge.
A voltage applied to the above-described contact electrostatic charging member is only a DC bias (to be referred to as xe2x80x9cDC electrostatic chargexe2x80x9d hereinafter) or an AC bias superimposed on a DC bias (to be referred to as xe2x80x9cAC electrostatic chargexe2x80x9d hereinafter). In general, AC electrostatic charge can realize uniform electrostatic charge in comparison with DC electrostatic charge.
In AC electrostatic charge, a DC voltage is superimposed as an application bias on an AC voltage twice or more the discharge start voltage by using a roller-shaped electrostatic charging member (electrostatic charging roller) as an electrostatic charging member (Japanese Patent Application Laid-Open Nos. 63-149669 and 1-267667). Alternatively, a DC voltage is superimposed as an application bias on an AC voltage twice or less than discharge start voltage by using a conductive brush as an electrostatic charging member (Japanese Patent Application Laid-Open No. 6-130732).
The above-described contact electrostatic charging method rarely produces ozone, requires a smaller number of building components of the electrostatic charging apparatus, and can provide a low-cost electrostatic charging apparatus. However, contact electrostatic charge greatly damages the electrophotographic photosensitive member, compared to corona electrostatic charge. This trend is prominent particularly in the use of an OPC photosensitive drum.
Even in the same contact electrostatic charge method, the damage to the electrophotographic photosensitive member changes depending on the application voltage to the electrostatic charging member. The damage to the electrophotographic photosensitive member is larger for a higher application voltage. Also when only a DC voltage is applied as an electrostatic charge bias, the damage increases in comparison with the case of rotating the photosensitive drum without applying any electrostatic charge bias. If an AC voltage is superimposed and applied as an electrostatic charge bias, the damage (particularly the shaving amount of the OPC photosensitive drum) further increases to about several times that in the case of applying only a DC voltage as an electrostatic charge bias.
Especially application of an AC voltage twice or more the discharge start voltage significantly increases the damage to the electrophotographic photosensitive member. Even at an AC voltage twice or less the discharge start voltage, the damage is large about several times that in application of only a DC voltage.
A higher frequency of an AC voltage applied as an electrostatic charge bias also tends to increase the damage to the electrophotographic photosensitive member (particularly the OPC photosensitive drum).
As described above, in recent years, a single-component developer is carried by a developing roller serving as a developer carrying member, and brought into contact with a photosensitive drum to develop an electrostatic latent image on the photosensitive drum. Contact rotation of the developing roller also shaves the photosensitive drum.
Generally, in the use of a single-component non-magnetic developing apparatus in which the developing roller comes into contact with the photosensitive drum, the peripheral speed of the developing roller is set higher than that of the photosensitive drum in order to ensure a necessary density. In particular, a developing roller having a relative peripheral speed ratio to the photosensitive drum tends to increase the damage to the photosensitive drum.
However, the rotation period of the photosensitive drum and the contact period between the photosensitive drum and the developing roller are not proportional to each other in a color image forming apparatus for switching developing apparatuses of a plurality of colors and developing an electrostatic latent image on a photosensitive drum, or an image forming apparatus which adopts a method of arranging a contact/separation mechanism for separating a developing roller from a photosensitive drum and of separating the developing apparatus from the photosensitive drum during rotation of the photosensitive drum in a non-image forming period in order to prevent any fog in contact developing.
As is apparent from the above description, the damage to the photosensitive drum changes depending on electrostatic charge conditions in an image forming apparatus having an electrostatic charging means for electrostatically charging the photosensitive drum by, e.g., contact electrostatic charge under a plurality of electrostatic charge conditions during image formation. The life of the photosensitive drum is difficult to accurately predict by a conventional method of detecting the life of the photosensitive drum simply by the number of turns of the photosensitive drum.
Also in the use of a developing apparatus which can be brought into contact with or separated from the photosensitive drum, the rotation period of the photosensitive drum and the contact period between the photosensitive drum and the developing roller are not proportional to each other. Thus, the life of the photosensitive drum cannot be accurately detected by a conventional method of detecting the life of the photosensitive drum simply from the number of turns of the photosensitive drum.
When, for example, the resolution is switched to high one with several process speeds, an image is formed by decreasing the process speed without changing the rotational speed of an exposing apparatus using a polygon mirror, or an image is reliably fixed by decreasing the process speed in order to thermally fix the toner on a thick sheet or the like.
In this manner, in an image forming apparatus having a process speed switching mode, the damage to the photosensitive drum changes upon a change in speed. Hence, the accurate life of the photosensitive drum cannot be detected.
A cartridge replacement warning may fail though the life of the photosensitive drum has expired and an image error occurs due to the above reasons. Alternatively, a cartridge replacement warning may be issued though the life of the photosensitive drum does not expire.
It is a principal object of the present invention to provide an image forming apparatus capable of accurately detecting that the life of an image bearing member has expired or will expire soon, an image bearing member life detecting method, and a cartridge detachably attachable to the image forming apparatus.
It is another object of the present invention to provide an image forming apparatus capable of accurately notifying the user that the replacement time has come or is coming soon on the basis of the life of the image bearing member, an image bearing member life detecting method, and a cartridge detachably attachable to the image forming apparatus.
It is still another object of the present invention to provide an image forming apparatus capable of accurately detecting that the image bearing member has expired or will expire soon particularly when an image is formed at a plurality of process speeds, the image bearing member is electrostatically charged under a plurality of electrostatic charge conditions, or the developing means can be separated from or brought into contact with the image bearing member, an image bearing member life detecting method, and a cartridge detachably attachable to the image forming apparatus.